LONDON - Chinese scientists have for the first time found strong evidence of how humans became infected with a new strain of bird flu: from chickens at a live market.

Chinese scientists compared swabs from birds at markets in eastern China to virus samples from four patients who caught the new H7N9 virus. The scientists found the virus from one patient was nearly identical to one found in a chicken. The research was published online Thursday in the journal Lancet.

FILE - In this April 3, 2013 file photo, chickens are seen at a chicken farm on the outskirts of Shanghai, China. Chinese scientists have for the first time found strong evidence of how humans got infected with a new strain of bird flu: from chickens at a live market. The research was published online Thursday April 25, 2013 in the journal Lancet. (AP Photo/Gillian Wong, File)

Finding definitive proof of how patients were infected is very difficult and experts have so far struggled to find much virus in birds. Despite taking nearly 48,000 samples from animals in live markets, Chinese officials found only 39 positive tests for H7N9. Experts had suspected birds in live markets to be the source of infection but weren't sure if other animals or wild birds might also be responsible.

Health officials have so far refrained from recommending any wide-scale slaughter of poultry to contain the disease, one of the main tools used previously to combat another bird flu strain, H5N1. Unlike that strain, H7N9 doesn't appear to sicken chickens, giving experts fewer signs as to where it might be spreading.

Chinese authorities have shut down live poultry markets in many affected regions, which seems to have slowed down the virus. Still, Taiwan reported its first case earlier this week.

So far, H7N9 has infected more than 100 people in China and killed more than 20.

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